!08 



HARDWICKE'S SCIENCE-GOSSIP. 



activity is probably as great as that of most insects, 

 there appears to be no occluding apparatus. ■ 



If there is no current of air in the finer tracheae, 

 how is the substitution of oxygen for carbonic acid 

 effected? The late Professor Graham showed how 

 Liy a difTusion-process the carbonic acid produced in 

 the remote cavities would be moved along the 

 smaller tubes and emptied into wider tubes, from 

 which it could be expelled by muscular action. The 

 carbonic acid is not merely exchanged for oxygen, 

 but for a larger volume of oxygen (O 95 : C Oj 81) ; 

 and there is consequently a tendency to accumulation 

 within the tubes, which is counteracted by the 

 ■elasticity of the air-vessels, as well as by special 

 muscular contractions.* 



The occluding apparatus has, no doubt, its proper 

 function, which is probably that of excluding water, 

 dust, and noxious gases from the tracheal system. It 

 may be found possible to apply a direct test to 

 Landois' explanation by observing whether in living 

 insects the occluding apparatus works rhythmically, 

 ^s, upon his hypothesis, it should do. 



The respiratory activity of insects varies greatly. 

 Warmth, feeding, and movement are found to 

 increase the frequency of their respirations and also 

 ihe quantity of carbonic acid exhaled. In Liebe'sf 

 experiments a Carabus produced "24 mgr. of carbonic 

 acid per hour in September, but only '09 mgr. per 

 hour in December. A rise of temperature raised 

 the product temporarily to twice its previous amount ; 

 but when the same insect was kept under experiment 

 for several days without food, the amount fell in 

 .spite of increased warmth. TreviranusJ gives the 

 carbonic acid exhaled by a humble-bee as varying 

 from 22 to 174, according as the temperature varied 

 from 56° to 74° F. 



Larva; often breathe little, especially such as lie 

 buried in wood, earth, or the bodies of other animals. 

 The respiration of pupoe is also sluggish, and not a 

 few are buried beneath the ground or shrouded in a 

 dense cocoon or pupa-case. Muscular activity 

 originates the chief demand for oxygen, and 

 accordingly insects of powerful flight are most 

 energetic in respiration. 



The question has been raised whether the respira- 

 tory movements are voluntary, or not. They continue 

 after decapitation, and may be observed even in the 

 detached abdomen of insects whose abdominal 

 nervous centres are not concentrated and fused with 

 those of the thorax. Plateau § observes that in the 

 abdomen after severance the respiratory movements 

 are stimulated or restrained by the same external 



* "Phil. Mag.," 1833. Reprinted in "Researches," p. 44. 

 Graham expressly applies the law of diffusion of gases to ex- 

 plain the respiration of insects. Sir John Lubbock quotes and 

 comments upon the passage in his paper on the Distribution of 

 the Tracheae in Insects (Linn. Trans, vol. xxiii.), but it has 

 dropped out of sight in the more recent discussions. 



+ Ueb. d. Respiration der Tracheaten. Chemnitz {1872). 



X See Table in Burmeister's " Manual," Eng. trans., p. 398. 



^ Rech. exp. sur les mouvements respiratoires des Insectes 

 (JS82). 



agents as in the uninjured animal. This points to a 

 nervous control independent of will or consciousness. 

 The voluntary and conscious life may nevertheless 

 modify actions which it does not originate ; and that 

 this is the case with the respiratory movements of 

 insects may be inferred from the-fact that they can be 

 checked, stopped, or restricted to a single segment, 

 apparently at the pleasure of the uninjured animal. 

 The respiratory centres seem to be the ganglia of the 

 respiratory segments. 



A rise of temperature proportionate to respiratory 

 activity has been observed in many insects. New- 

 port* tells how the female humble-bee places herself 

 on the cells of pupce ready to emerge, and acce- 

 lerates her inspirations to 120 or 130 per minute. 

 During these observations he found, in some instances, 

 that the temperature of a single bee was more than 

 20° above that of the outer air. 



Some insects can remain long without breathing. 

 They survive for many hours when placed in an 

 exhausted receiver, or in certain irrespirable gases. 

 Cockroaches in carbonic acid speedily become 

 insensible, but after twelve hours' exposure to the 

 pure gas they revive, and appear none the worse, 

 H. Miillerf says that an insect, placed in a small, 

 confined space, absorbs all the oxygen. In Sir 

 Humphry Davy's "Consolations in Travel "J is a 

 description of the Lago dei Tartari, near Tivoli, a 

 small lake whose waters are warm and saturated with 

 carbonic acid. Insects abound on its floating islands ; 

 though water-birds, attracted by the abundance of 

 food, are obliged to confine themselves to the banks, 

 as the carbonic acid disengaged from the surface 

 would be fatal to them, if they ventured to swim 

 upon it when tranquil. 



Kowalewsky, Biitschli, and Hatschek have de- 

 scribed the first stages of development of the tracheal 

 system. Lateral pouches form in the integument ; 

 these send out anterior and posterior extensions, 

 which anastomose and form the longitudinal trunks. 

 The tracheal ramifications are not formed directly 

 by a process of invagination, but by the separation of 

 chitinogenous cells, which cohere into strings, and 

 then form irregular tubules. The cells secrete a 

 chitinous lining, and afterwards lose their distinct 

 contours, fusing to a continuous tissue, in which the 

 individual cells are indicated only by their nuclei, 

 though by appropriate reagents the cell-boundaries 

 can be defined. 



It \\7!£. been held that the spiracles correspond 

 morphologically with tracheal gills, and that they are 

 the scars or broken ends of tubes, which in other 

 cases project beyond the body as respiratory appen- 

 dages. Palmen seems, however, to have proved 

 that there is no constant relation of spiracles to 



* Art. " Insecta," Cyc. Anat. and Phys., p. 989. 

 t Pogg. Ann. 1872, Hft. 3. 



X Works, vol. ix. p. 287. Ihis passage has been cited by 

 Rathke. 



